The present disclosure is generally directed, in various embodiments, to imaging members. More particularly, the disclosure relates to various embodiments of an imaging member for liquid xerography comprising an optional a substrate, an optional undercoat layer, a charge generation layer, a charge transport layer, and an optional overcoat layer. In embodiments one or more of the optional substrate, the optional undercoat layer, the charge generation layer, the charge transport layer, and an optional overcoat layer comprise a structure organic film.
A typical electrostatographic printing machine employs a photoconductive member that is sensitized by charging the photoconductive member to a substantially uniform potential. The charged portion of the photoconductive member is image-wise discharged by light to form a latent image of an original image on the photoconductive member. Exposing the charged photoconductive member with light selectively dissipates the charge to form the latent image on the charged photoconductive member. The latent image recorded on the photoconductive member is developed using a developer material. The developer material can be a liquid developer material known in the literature as “liquid electrophoretic ink” or simply “liquid ink” or “liquid xerographic toner” or simply “liquid toner” or “liquid immersion development.” In a liquid development system, the photoconductive surface is contacted by liquid developer material comprising finely divided toner particles dispersed in an insulating liquid carrier. The latent image attracts the toner particles dispersed throughout the insulating liquid carrier material particles to the photoconductive surface to develop the latent image, thus forming a visible image.
Liquid toners have many advantages and often produce images of higher quality than images formed with powder toners. For example, images developed with liquid toner may adhere to the copy substrate without requiring fixing or fusing to the copy substrate. Thus, the liquid toner may not need to include a resin for fusing purposes. In addition, the toner particles suspended in the liquid carrier material can be made significantly smaller than the toner particles used in powder toners. Using such small toner particles is particularly advantageous in multicolor processes where multiple layers of toner particles generate the final multicolor output image. An additional advantage of liquid toners is that the particles are charged by a controlled chemical reaction between the sites on the particle surface and molecules dissolved in the liquid carrier material. This charging makes possible liquid toner particles with 20-50% pigment, instead of the 2-10% pigment, which is common in dry toner particles. This increased pigment loading reduces the amount of resin contained in the image transferred to the final printed substrate. This reduced resin reduces paper curl and leads to multicolor output images, which generally have a significantly more uniform finish compared to images formed using powder toners.
Liquid toners typically contain about 1-5% by weight of fine solid particulate toner material disbursed in the liquid carrier material. The liquid carrier material is typically a hydrocarbon. After developing the latent electrostatic image, the developed image on the photoreceptor may contain 6-25% by weight of the solid particulate toner particles along with residual liquid hydrocarbon carrier. To complete the development process, the solid particulate toner material is typically compacted onto the photoreceptor and the excess liquid carrier material removed from the photoreceptor.
Liquid toner development systems are generally capable of very high image resolution because the toner particles can safely be ten or more times smaller than dry toner particles. Typical dry toner particles are on the order of 10 microns in diameter. Typical liquid toner particles are on the order of 1 micron in diameter. Liquid toner development systems show impressive grey scale image density response to variations in image charge and achieve high levels of image density using small amounts of liquid developer.
However, internal cyclic life associated with imaging members for liquid xerography sometimes is not good enough due to the lack of solvent resistance and electrical performance of the imaging members over time. It has been found that typical image members, such as photoreceptors, which may be acceptable for use with dry toners, become unstable when employed with liquid development systems. These imaging members (photoreceptors) suffer from “physical damage.” The term “physical damage” refers for example damage, which optionally may be visually detected, such as cracking, crazing, crystallization of active compounds, phase separation of activating compounds and extraction of activating compounds caused by contact with the organic carrier fluid, such as isoparaffinic hydrocarbons e.g. isopar, commonly employed in liquid developer inks which, in turn, markedly degrade the mechanical integrity and properties of the layer, such as a photoreceptor. More specifically, the organic carrier fluid of a liquid developer tends to leach out activating small molecules, such as the arylamine containing compounds typically used in the charge transport layers. Representative of this class of materials are: N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine; bis-(4-diethylamino-2-methylphenyl)-phenylmethane; 2,5-bis-(4′-dimethylaminophenyl)-1,3,4,-oxadiazole; 1-phenyl-3-(4′-diethylaminostyryl)-5-(4″-diethylaminophenyl)-pyrazoline; 1,1-bis-(4-(di-N,N′-p-methylphenyl)-aminophenyl)-cyclohexane; 4-diethylaminobenzaldehyde-1,1-diphenylhydrazone; 1,1-diphenyl-2(p-N,N-diphenyl amino phenyl)-ethylene; N-ethylcarbazole-3-carboxaldehyde-1-methyl-1-phenylhydrazone. The leaching process results in crystallization of the activating small molecules, such as the aforementioned arylamine compounds, onto the photoreceptor surface and subsequent migration of arylamines into the liquid developer ink. In addition, the ink vehicle, typically a C10-C14 branched hydrocarbon, induces the formation of cracks and crazes in the photoreceptor surface. These effects lead to copy defects and shortened photoreceptor life. The degradation of the photoreceptor manifests itself as increased background and other printing defects prior to complete physical photoreceptor failure.
The leaching out of the activating small molecule also increases the susceptibility of the transport layer to solvent/stress cracking when the belt is parked over a belt support roller during periods of non-use. Some carrier fluids also promote phase separation of the activating small molecules, such as arylamine compounds and their aforementioned derivatives, in the transport layers, particularly when high concentrations of the arylamine compounds are present in the transport layer binder. Phase separation of activating small molecules also adversely alters the electrical and mechanical properties of a photoreceptor. Although flexing is normally not encountered with rigid, cylindrical, multilayered photoreceptors which utilize charge transport layers containing activating small molecules dispersed or dissolved in a polymeric film forming binder, electrical degradation are similarly encountered during development with liquid developers. Sufficient degradation of these photoreceptors by liquid developers can occur in less than eight hours of use thereby rendering the photoreceptor unsuitable for even low quality xerographic imaging purposes. Thus, in advanced imaging systems utilizing belt photoreceptors exposed to liquid development systems, cracking and crazing have been encountered in critical charge transport layers during belt cycling. Cracks developing in charge transport layers during cycling can be manifested as print-out defects adversely affecting copy quality. Furthermore, cracks in the photoreceptor pick up toner particles, which cannot be removed in the cleaning step and may be transferred to the background in subsequent prints. In addition, crack areas are subject to delamination when contacted with blade cleaning devices thus limiting the options in electrophotographic product design.
As such, new imaging members for liquid xerography that do not suffer from the above problems and exhibit improved properties such as stability, processing convenience, longer internal cyclic life, and longer operational life etc. are needed.